*update* a 27mm seatpost fits perfectly so it seems more likely a 531st. Should still be a nice and light ride once built though!According to Tony Oliver's book, 531c is 0.8/0.5mm single butted, while 531ST is 1.0/0.7mm.
Both 28.6mm OD. In his seat tube chart it recommends 27.2 for 531c and 27.0 for ST.
It looks to me as though the seat tube clamp is a little tight on your frame (the slot looks narrow), if you carefully open it up so the sides of the slot are parallel it will help decide which fits correctly. The seatpost should move smoothly in like a piston, and not rattle even when the clamp is loose.